This invention relates to a method and apparatus for establishing humidity gradients within a single-zone air conditioned space. More particularly, the present invention relates to a method and apparatus for modulating humidity across large single-zone air conditioned spaces such as those typically found in supermarkets.
Supermarkets are highly intensive energy operations. Energy cost represents a significant share of overall operating cost, often equaling a store's annual profit. The largest share of supermarket energy cost is for refrigeration. Display cases refrigerated 24 hours a day typically account for more than half the electricity used in the store. Excess humidity causes the refrigeration system to consume more energy. Optimum dehumidification can help the efficiency of the refrigeration system and reduce the associated energy cost. In most commercial HVAC applications, the primary function of an air conditioning system is temperature control. In supermarkets, however, the emphasis is on dehumidification, because reducing the amount of moisture in the air causes the refrigeration system to operate more efficiently.
Once a lower humidity level is achieved in the supermarket, a number of operational benefits are simultaneously achieved. First, the energy expended by the refrigeration cases in removing moisture from the air is reduced. Second, the buildup of frost on the refrigeration coils is reduced, thereby reducing the insulating effect of frost on the coils and allowing the coils to be defrosted less frequently. Third, the need for anti-sweat heating of display case doors and other surfaces is reduced. In addition to reduced energy use for the anti-sweat heaters themselves, the load on the refrigerating coil is also reduced because less heat is transferred from the anti-sweat heaters into the display case.
An air conditioning zone is a space enclosed or separated from other spaces or environments. Traditionally, air conditioned zones are bounded by fixed walls or other physical separations. Such zones may also be bounded by flexible membrane barriers or high velocity streams of air known as "air curtains". System designers have heretofore recognized that temperature gradients, caused by internal heat generating sources such as lights, electrical equipment or refrigeration devices, may develop within such zones. Typically, the refrigeration cases in a supermarket are located some distance from the fresh produce section of the sales area. The ambient temperature in the area immediately surrounding the refrigeration cases is usually lower than the temperature in the other areas of the store and is often below a customer's comfort level. In the remainder of the store, temperature levels are generally acceptable, with the exception of the checkout area. Temperatures rise in the checkout area because windows, entryways, and concentrations of customers and employees are typically located there. It is also generally recognized that temperature gradients may result from vertical stratification of warmer air. To counteract these gradients and achieve temperature uniformity, return ducts located near the heat generating source and air circulation equipment such as ceiling fans are typically employed.
In contrast to temperature gradients, it has generally been believed that significant humidity gradients do not and cannot exist within a single zone. This belief rests in part on the rate with which moisture diffusion is thought to occur within such zones. As a result of this belief, the space conditioning control strategy recommended in professional literature specifies that large single zones such as supermarkets should be treated as a single entity, wherein fixed set points for temperature and humidity are maintained throughout the space. These set points are almost uniformly specified as 75.degree. F. dry bulb temperature and 55% relative humidity. The operating condition defined by these set points is so well accepted by design and operating personnel in the supermarket industry that all equipment designed for the conditioned space (sales area) is rated at that operating condition. In fact, capacity and power consumption values for refrigerated cases are not published for other operating conditions. Moreover, since conventional air conditioning systems are intended primarily for temperature control, they produce relative humidities approximating the 55% level typically employed in supermarket applications. Such systems are not designed to produce lower humidity levels.
Because of the increased cost of electric power and the concern for the availability of electric power in the future, system designers and engineers have investigated the advantages of other set points. In applications such as supermarkets, wherein refrigeration cases are located within the conditioned space, significant power savings can be realized from the operation of the refrigeration cases if the ambient humidity is lessened to 30%. As explained above, this power savings results from the fact that it takes a refrigeration case less energy to cool dryer air, the latent load of such air having been reduced by the lower ambient humidity. Unfortunately, in the supermarket application, a lower overall humidity level within the conditioned space is unacceptable, because lower humidity levels have an adverse effect on fresh produce. Where the humidity is too low, vegetables begin to wilt--requiring spraying, which acts to raise the humidity again. This condition forces system designers to opt for an overall ambient humidity level of 55%--which is not optimal for the operation of the refrigeration cases.
When conventional electric systems have been employed to control humidity in supermarkets, their performance bas been less than satisfactory. When the system is operated long enough to achieve the desired 55% relative humidity level, the air in some or all of the store often becomes too cool, thus requiring heating to achieve a comfortable ambient temperature level. Several technologies, including gas fired desiccant systems and high efficiency air conditioning systems, have been adapted and developed to help supermarket owners efficiently achieve the desired 55% relative humidity level.
Gas fired desiccant systems, which were originally developed for sensitive product shipping and warehousing applications, remove moisture from the air to achieve a lower humidity level. In recent years, this technology bas been combined with conventional electric air conditioning systems for use in supermarkets. In such systems, the desiccant system first acts to dehumidify return air from the zone. Since the desiccant system also works to warm air passing through, this added heat must next be removed by electric air conditioning before the air can be passed back into the zone. The heat added by the desiccant equipment represents an additional load for the electric air conditioning system in addition to the space cooling load.
High efficiency electric air conditioning technologies cool return air to lower temperatures--approximately 40.degree. to 45.degree. F.--in order to remove moisture. In these systems, only a percentage of the return is cooled. More particularly, enough of the return air is cooled to achieve the required low humidity level. The remainder of the return air is allowed to bypass the cooling coil, thereby minimizing overcooling and the need to re-heat the conditioned air for its return to the store.
Different air flow techniques have also been employed in connection with these new technologies to further improve system performance. In a supermarket, much of the air returning the air conditioning system from within the store may already be cool as well as low in humidity. For example, to avoid uncomfortably cold aisles, the cold, dry air escaping from refrigeration display cases is typically captured by returns under the cases and returned to the air conditioning unit. In comparison with outside air, air returned from elsewhere in the store is also relatively cool and dry. Although such air does not require significant processing, conventional air conditioning systems channel it through the cooling and dehumidification process just as if it were warm and humid air taken from outside the store. Modern airflow techniques address these inefficiencies by channeling return air so as to bypass the cooling and dehumidification units.
In one such channeling technique known as a single path system, the cooling unit can be sized for the smaller volume of air which will actually pass through the unit. After that air is cooled to the low temperature needed to reach the desired humidity, it is mixed with the bypassed air. This blend is typically cooler than the conditioned air normally delivered by conventional air conditioning systems, so less of it is needed to achieved the desired store temperature (750.degree. F.) and humidity (55%).
An alternative air channeling technique is known as dual path channelling. In the dual path system, the air is processed in two separate streams, with the outdoor air directed to a primary coil and the relatively cool and dry return air being cooled by a secondary coil only when necessary. Both the single and dual path systems allow system designers to employ smaller cooling units and circulation fans, thereby effecting significant energy savings. Other system enhancements which have been added to improve performance in the supermarket industry include heat pipe exchange and ice storage systems.
All of the above techniques share the common goal of maintaining a uniform temperature (75.degree. F.) and humidity (55%) throughout the air conditioned zone. Although significant energy savings could result if the ambient humidity in the area around the refrigeration cases was lowered to 45%, no system to date has successfully capitalized on this fact because an overall lower humidity level throughout the store is undesirable for certain goods such as fresh produce, and achieving such gradients has, in practice, proven difficult to achieve.